Do not reference the template property on a view instance after initialization.

This PR implements one of the solutions proposed to the problem outlined in #307.

During initialization the template property at the instance level is deleted: https://github.com/tbranyen/backbone.layoutmanager/blob/master/backbone.layoutmanager.js#L736

Continuing to reference the template property on the view instance can cause problems because `delete` will not recursively remove the property, up the prototype chain. 

I've also wrapped `options.template` in `_.result()` so that the template can be dynamically determined during the render process.